A battery system for warming up a battery is proposed by, for example, JP 2006-6073A. This conventional system has a boost converter unit, which converts a voltage level of charging and discharging a battery, so that a ripple current flowing in and out the battery is increased when the temperature of the battery is in a low temperature range lower than a predetermined reference temperature. When the ripple current flows in and out the battery, the electric power consumed by an internal resistance of the battery is increased and hence the battery temperature is raised quickly. When the battery temperature is thus increased, the internal resistance is lowered and hence the output power of the battery is suppressed from decreasing under low temperatures.
According to the conventional system, when charge and discharge of the battery are performed by the input and output of the ripple current relative to the battery at high frequencies between the battery and the boost converter unit, various components, for example, electric wires and the boost converter unit, other than the battery will also radiate heat of Joule heat. The heat radiated by such components is wasted externally without being recovered. Thus, the heat is not efficiently used within the system.